This invention relates to halogen-containing polymers. More particularly this invention relates to the stabilization of such polymers against deterioration caused by heat, light or mechanical processing.
Polymers and copolymers of vinyl chloride are thermoplastics which contain labile chlorine atoms which are unstable to heat. Therefore when such polymers are heated during normal fabricating processes, such as extrusion, calendering and injection moulding, they tend to decompose fairly rapidly with the evolution of hydrogen chloride gas. The evolution of hydrogen chloride gas is accompanied by unsaturation being formed on the polymer backbone and the resulting polyene as it is built-up, causes the characteristic colour change on degradation through yellow, through amber, through black. This change in colour is accompanied by loss in mechanical properties. This problem has been overcome by the plastics industry by the addition of small quantities of additives to achieve acceptable stability such that the polymer may be processed.
A wide variety of materials have been proposed as stabilisers for polyvinylchloride, and several distinct classes of compound have emerged. For example basic inorganic lead salts form one class which although cheap and effective have the disadvantages of high toxicity, high opacity, and a tendancy to stain in industrial atmospheres. Metal soaps of metals of groups two and three of the periodic classification form another class of compounds which are widely used in complex formulations together with epoxy compounds, antioxidants and chelators. This class of compound is used in many applications and can if required produce clear polyvinylchloride. However the heat stability imparted by this class of compound is still limited and new disadvantages are introduced such as poor plateout. The most powerful class of polyvinylchloride stabiliser to emerge are organotin compounds. Of particular value are the dialkyltin compounds, particularly when combined with a mercaptide anion.
Typical of the organotin stabilisers in wide use in industry today are dibutyltin bis lauryl mercaptide and dibutyltin bis iso-octyl thioglycollate. The latter compound has enjoyed wide use in industry for many years.
In recent years however, the trend towards higher output rates from a given piece of equipment together with new processing techniques involving high temperatures and high mechanical shear have exposed the limitations of such organotin stabilisers.